Jupiter is made up of a majority of hydrogen
and helium gas, so there is no real surface to
stand on. If you were to try and stand on the
surface, you would sink until you reached a solid
core at the center of the planet.

Answer 2:

No, we could not stand on Jupiter. Jupiter
has a solid core, but it's under so much
pressure that the atoms of your body would be
crushed into a metal were you to get that deep
(note: this is not nuclear fusion; it's called
electron degeneracy, and is similar to what
happens inside of a white
dwarf star ).